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Noisy return

Have two returns for 2.5 ton furnace , one 8x20 , other 10 inch round. They both go to return plenum with 24x20 filter box.
Its very loud noise coming from 12x12 return grill (10" return duct) then furnace is on . 10" round is about 25' long with four 90ties. Take off is 7x14.
Any ideas ?
I took filter panel off - same thing. i took furnace cover to expose fan - same thing. Something is wrong with this duct?
Thanks

Have two returns for 2.5 ton furnace , one 8x20 , other 10 inch round. They both go to return plenum with 24x20 filter box.
Its very loud noise coming from 12x12 return grill (10" return duct) then furnace is on . 10" round is about 25' long with four 90ties. Take off is 7x14.
Any ideas ?
I took filter panel off - same thing. i took furnace cover to expose fan - same thing. Something is wrong with this duct?
Thanks

Have two returns for 2.5 ton furnace , one 8x20 , other 10 inch round. They both go to return plenum with 24x20 filter box.

Its very loud noise coming from 12x12 return grill (10" return duct) then furnace is on a 10" round is about 25' long with four 90ties. Take off is 7x14.
Any ideas ?
I took filter panel off - same thing. i took furnace cover to expose fan - same thing. Something is wrong with this duct?
Thanks

Is there more than one Return air grille?

If there is more than one RA grille; what percentage goes through the noisy 12X12? That is only one sq.ft., of open air area without deducting the area the grille blades block.

If that were the only RA grille the velocity through that grille would be well over 1000-fpm; that would be over twice the velocity needed, as 400 to 500-fpm limit is required to reduce airflow noise; I'd prefer grille sizing so the velocity through the grille area(s) would not go above 450-fpm.

The system needs to move 1000-cfm @500-fpm or less through the RA grilles for a 2.5-ton A/C.

311-cfm + 685-cfm = 996-cfm @500-fpm velocity through those grilles. Grilles will work if enough room behind them so there is no restriction.

According to manual D initial velocity "through a cheap media filter" should be 300-fpm.

When you figure using .62% for reduced free air filter area; the free air area (Ak) is 2.06 * 300-fpm = will only flow 620-cfm; therefore it could also use a larger filter area, however...

When you divide 1000-cfm by 2.06-Ak the result is 485-fpm which is better than a lot of filter areas; so you might let it pass.

If the other duct feeds into the 10" flex duct moving 1000-cfm is way too small; optimally it needs a 16" flex duct to get the velocity down to a little over 700-fpm. It's late & I didn't verify that situation...

With a sheet metal RA duct, I'd still go with the 16" RA duct, the friction rate between flex & metal goes from 0.06 for flex to 0.02 for metal @1000-cfm of airflow volume. Either way @1000-cfm of airflow the velocity is 717-fpm.

However, if it is flowing 1200-cfm as George2 believes, then, with (2) 90's, over 25' the friction loss is 0.03"; over 100' it would be 0.08" friction loss & either way the fpm velocity is 860. The standard is to not go over 800-fpm velocity on a Return Air duct.